Hold the barbell with palms facing away from you and just wider than your shoulders. Feet apart and tense your core pulling your ribcage down to your belly button, but still keeping your body upright. This bit is really important if you want to isolate the shoulders.
push the bar directly above the shoulders, using only the muscles in your arms and shoulders. At the end position your arms should be straight and knuckles facing the ceiling.
Once you max out, if you want to cheat a few reps then bend your knees with the bar at the bottom of the rep and then as you push the bar up in an explosive action, straighten your legs at the same time.